The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A wide variety of coated particles are known. They are needed if a particle having particular properties, such as a given luminescence, absorption, color etc., lacks other properties, which are also needed, however. Thus, biological markers can be provided by particles which are particularly readily detected due to their luminescence and easily undergo binding with specific substances, such as enzymes or the like, due to the properties of their shell material. The shells are often organic, which limits applicability.
Also, in particular applications, only a few properties of particles are desired while other properties are disturbing. Thus, there are a wide variety of transparent IR absorbers which at the same time have a good electric conductivity (“transparent conductive oxides”, TCO). Often, this conductivity is even retained when particles are incorporated in a matrix, such as a paint, which is the case even if the percolation limit is not reached. This is often undesirable, such as with glazing, because the two-dimensional application on glazing results in conductive surfaces, which adversely affects mobile phones, for example, in a building or house. In such cases, the TCOs or other substances are used as aggregates.
Aggregates, such as additives, fillers, pigments and the like, are required in a wide variety of applications. They confer particular properties, such as a particular desired optical behavior, to a matrix in which they are incorporated or to a larger body to which they adhere chemically or physically. It is important that while the aggregates must provide the desired properties, they must not have negative effects, such as with respect to the stability or biocompatibility or the biological safety and/or safety under food laws.
This is problematic especially when the aggregates cannot be incorporated firmly and at least essentially inaccessible in a matrix, and the material with which the aggregate is to be used additionally is exposed to changing and/or chemically aggressive environments. They may be exemplified by aggregates for fibrous materials, such as cotton. There are aggregates which must remain adhered to the cotton fibers even though the material is exposed to, for example, the acidic environment of transpiration and the like. The same applies to aggregates for, for example, paper fibers, cellulose and the like; there may be mentioned, for example, titanium oxide aggregates as whitening agents. Also problematic are aggregates, such as for printing and other inks, mainly if the printed matter frequently comes into contact with skin, because incorporation in printing inks is not particularly stable as a rule.
An aggregate with a high attractiveness because of its optical properties is indium tin oxide with an Sn content of 7±0.5 mole percent designated for printing in offset and ink-jet printing processes. In order to render this material more attractive beyond its optical properties, it is required to prepare it in a biocompatible form. Here, “biocompatible” does not necessarily mean “safe in terms of food technology”, but nevertheless in a form which renders the contact with such materials harmless in accordance with the definition below.
Problems in terms of production technology occur, in particular, when the biocompatibility of pigments to be printed is to be improved, or if pigments having an improved biocompatibility are to be prepared for use in printing without significantly influencing desired pigment properties, such as the particle size. Often, different production processes are required from pigment to pigment in order to obtain desired optical properties and printing properties with sufficient biocompatibility.
Relevant pigments include, for example, indium tin oxide with a tin content of 7±0.5 mole percent and a particle size of smaller than 10 μm. Efficient processes for producing such pigments in a biocompatible form are desirable.
There are a number of property rights which already deal with aggregates. There may be mentioned, for example, EP 0 492 223 A2 which relates to silanized pigments and their use for inhibiting the yellowing of pigmented plastic materials in which the increasing of the stability of pigment surfaces towards the action of air, oxygen, heat and light is addressed, and the chemisorption of silane compounds to pigments is mentioned, wherein the pigment coating is to be effected, in particular, without adding solvents or adding other substances, such as coupling agents or carrier liquids, in an intensive mixer. Further, there may be mentioned DE 198 17 286, which relates to a multilayered pearlescent pigment on the basis of an opaque substrate, wherein this application discusses, inter alia, the pigmenting of papers for bonds and securities and of packagings, as well as the laser marking of polymeric materials and papers; as metal oxides, there are mentioned TiO2, ZrO2, Fe2O3, Fe3O4, Cr2O3, ZnO, (SnSb)O2, Al2O3, mixtures thereof, SiO2.
In this specification, it is proposed to coat mica pigments having a particle size of from about 10 μm in such a way that they exhibit a particularly pronounced color flop, which means that the interference colors of the mica are to depend to a very high extent on the viewing angle. The use thereof in car paints is exemplified.
Further, there may be mentioned EP 0 608 388 B1, which discloses a plate-like pigment having a high luster and high opacity or high transparency, which is prepared in a particular way and provided with a matrix to achieve a luster. Surface-modified pigments in the form of titanium dioxide pigments and a layer of borates of alkaline earth metals and double borates of alkali and/or alkaline earth metals are disclosed in EP 0 641 842 B1. DE 697 23 347 relates to spherical SiO2 particles having a size of from 5 to 500 nm and coated at individual points with metal oxide particles having a size of less than 60 nm.
In addition, DE 100 22 037 A1 relates to IR-absorbing compositions containing transparent thermoplastic polymers and surface-modified oxide particles having a particle size of less than 200 nm and organic near-infrared absorbers, as well as their preparation, use and products prepared therefrom.
EP 0 245 984 A1 describes the coating of TiO2 particles with silicate. The addition of the silicate solution during the coating takes place at a pH value substantially above the isoelectric point of titanium oxide, without any input of additional energy. The pH value varies highly during the coating process. An uncontrollably rapid growth of the coating during the coating process and the coating of particle agglomerates cannot be prevented with this process, which is why the pigment formed must be comminuted again in a further step.
U.S. Pat. No. 6,440,322 B1 describes the coating of iron oxide particles with silicate. In this described process too, the pH value is not kept constant during the coating and lies far above pH 8 during the coating and is adjusted with hydrochloric acid to a pH of 8 after the reaction. No additional energy is input in this coating process either.
EP 1 477 465 A1 describes the coating of glass substrates with a coating material comprising particles of indium tin oxide and particles of silica. In the preparation of the coating material, the indium tin oxide particles are added without previous dissolution or dispersion to a mixture containing water glass and silica particles.
JP 2003-246965 describes the modification of particles of indium tin oxide with tetraethoxysilanes.
DE 697 08 085 T2 describes the coating of oxide particles with silicon dioxide, in which no additional energy input takes place and the coating occurs in a highly alkaline medium at a pH within a range of from 8 to 10. In this coating process, an additional electrolyte is further needed obligatorily. Since the coating takes place in a highly alkaline medium, an uncontrolled growth of the coating occurs. Therefore, the coated particles must be atomized after drying (cf. p. 8, 3rd paragraph).